Conventionally, as shown in an electric four-wheel drive vehicle, a well-known vehicle drive device in which front wheels are driven by an internal combustion engine and rear wheels are driven by an electric motor is commonly equipped with an electromagnetic clutch as an actuator for switching the transfer and non-transfer of the drive force between rear wheels and the electric motor for driving the rear wheels. The vehicle drive device is, for example, described in Japanese Patent Laid-open No. 2003-326997. Generally, a battery supplies electric power to the coil located in the electromagnetic clutch.
On the other hand, there is a well-known conventional vehicle drive device as described in Japanese Patent Laid-open No. 2003-079004. In the vehicle drive device, when a generator's output voltage is less than a prescribed voltage equivalent to a storage battery's output voltage, the electric power outputted from the generator's rectification circuit is boosted to a prescribed voltage by using a boost circuit, such as a DC/DC converter, and the amount of current generated at the low-speed start when the number of motor revolutions is few is increased, thereby high motor torque is obtained.
However, in the vehicle drive device described in Japanese Patent Laid-open No. 2003-079004, the voltage outputted by the generator is boosted, thereby causing the input voltage of the boost means to increase up to the maximum output voltage of the generator and also causing output voltage of the generator to change due to fluctuations of the load of the motor. As a result, there are problems such as increase in conversion noise, power loss and the size of the parts. Therefore, it is necessary to use a boost means which has variable input and output and can withstand high pressure.
Another well-known vehicle drive device is described in FIG. 4 of Japanese Patent Laid-open No. 2001-352795. In the vehicle drive device, the output of a power source such as a battery is boosted by a DC/DC converter and supplied to a field coil of the electric DC motor. This configuration keeps battery voltage almost constant, which eliminates the above problems and also makes it possible to increase the motor's output torque.